Studies are continuing on the proliferative patterns of the heteroploid cells of the methylcholanthrene-induced rat sarcoma as the model for heteroploid human solid tumors. The relation of the DNA contents and DNA synthetic rates and the effects thereon of commonly employed antitumor drugs is to be examined. In addition studies are under way on the chromosome replication patterns in control and in drug-treated animals. These investigations examine aspects of drug action and resistance in an animal model which has striking cytokinetic similarities to some human solid tumors.